1. Field of the Invention
The present invention relates to a self-pulsation type semiconductor laser which realizes multi-modes by causing self-pulsation.
2. Description of the Related Art
A semiconductor laser is used as a light source for an optical disk apparatus etc. At this time, how to suppress the so-called "returning light noise" generated by part of the light reflected from the optical disk striking the semiconductor laser is important.
As one type of semiconductor laser designed to suppress this returning light noise, there is known a so-called "self-pulsation type semiconductor laser" which realizes multi-modes by causing self-pulsation of the semiconductor laser.
FIG. 7 is a cross-sectional view of an example of the configuration of a self-pulsation type semiconductor laser of the related art.
Note that here, a case is shown where an AlGaInP based material is used for configuring the self-pulsation type semiconductor laser.
As shown in FIG. 7, a self-pulsation type semiconductor laser 10 is comprised of an n-type GaAs substrate 11 on which are successively stacked an n-type AlGaInP clad layer 12, a GaInP active layer 13, a p-type AlGaInP clad layer 14, a p-type GaInP intermediate layer 15, and a p-type GaAs cap layer 16.
The upper layer portion of the p-type AlGaInP clad layer 14, the p-type GaInP intermediate layer 15, and the p-type GaAs cap layer 16 have mesa-type stripe shapes extending in one direction.
Namely, a stripe portion 17 is formed by the upper layer of the p-type AlGaInP clad layer 14, the p-type GaInP intermediate layer 15, and the p-type GaAs cap layer 16.
An n-type GaAs current narrowing layer 18 is buried at the portions at the two sides of the stripe portion 17, by which a current narrowing structure is formed.
A p-side electrode 19 such as a Ti/Pt/Au electrode is provided on the p-type GaAs cap layer 16 and the n-type GaAs current narrowing layer 18.
On the other hand, an n-side electrode such as an AuGe/Ni/Au electrode is provided on the other surface of the n-type GaAs substrate 11.
FIG. 8 is a rough graph of the distribution of the refractive index of the self-pulsation type semiconductor laser 10 shown in FIG. 7.
Here, a refractive index distribution parallel to the direction of a pn junction of the self-pulsation type semiconductor laser 10 and perpendicular to a longitudinal direction of the resonator (hereinafter this direction will be referred to as a "horizontal direction") is shown in correspondence with FIG. 7.
As shown in FIG. 8, the self-pulsation type semiconductor laser 10 has a refractive index in the horizontal direction of a high refractive index n1 at a portion corresponding to the stripe portion 17 and of a low refractive index n2 at portions corresponding to the two sides of the stripe portion 17, that is, a so-called "step shaped" refractive index distribution.
In this way, in the self-pulsation type semiconductor laser 10, light is guided in the horizontal direction by changing the refractive index in the horizontal direction in a step manner.
In this case, a refractive index difference .DELTA.n (=n1-n2) between a portion corresponding to the stripe portion 17 and portions corresponding to the two sides of it is set to be not more than 0.003, whereby the confinement of light in the horizontal direction by the GaInP active layer 13 is eased.
During operation of the self-pulsation type semiconductor laser 10 configured as above, as shown in FIG. 7, a width WP of the optical waveguide region 22 becomes larger than a width WG of a gain region 21 inside the GaInP active layer 13. Thus, the optical waveguide region 22 outside the gain region 21 becomes a saturable absorbing region 23.
In the self-pulsation type semiconductor laser 10, the change of the refractive index in the horizontal direction is reduced so as to increase the amount of seepage of light in the horizontal direction and increase the interaction between the light and the saturable absorbing region 23 inside the GaInP active layer 13 so as to realize self-pulsation. Accordingly, it is necessary to secure a sufficient saturable absorbing region 23.
Turning now to the problem to be solved by the present invention, as explained above, the self-pulsation type semiconductor laser 10 of the related art has a so-called "ridge configuration" as shown in FIG. 9 and provides a saturable absorbing region (SAR) at the two sides of the optical waveguide inside the active layer to realize self-pulsation.
In this case, as shown in FIG. 9, when a gain region (a width of which is G) inside an active layer generated by the spread of the current is made as narrow as possible and the optical waveguide spot size (a width of which is P) is made conversely made relatively large to fulfill a relationship of P&gt;G, the amount of difference functions as a saturable absorbing region to generate self-pulsation.
For this, specifically, the refractive index difference .DELTA.n of the waveguide is made an intermediate guide of about 0.005 to 0.001 between an index guide and a gain guide to fulfill this relationship.
In such a semiconductor laser of the related art, however, since the width of the saturable absorbing region is determined by the delicate balance between the spread of light and the spread of current, there is instability such as a poor yield of the laser generating self-pulsation, a suppressed self-pulsation at the time of high temperature operation due to an increased current diffusion and narrower saturable absorbing region, and similarly a suppressed self-pulsation at the time of high output operation due to an increased current diffusion and narrower saturable absorbing region.
Especially, at a high temperature and high output, the so-called pulsation stops and the problem of noise arises.
Furthermore, a self-pulsation type semiconductor laser generally has a current threshold Ith considerably higher (about 1.5 times) that in an ordinary index guide type or a gain guide type. Also, depending on the system, sharp rising kinks are caused in the so-called L-I characteristic near the current threshold Ith. These have proven to be obstacles in applying the laser.